1. Field of Invention
The present invention relates to a method and apparatus for correcting electron-beam alignment and to a method and apparatus for setting the beam current of a camera tube.
2. Description of Prior Art
In a camera tube incident radiation is used to determine the current received per picture element on the camera tube target from a scanning electron beam. The electron beam is generated by a gun assembly within the camera tube and is aligned with the focus field by magnetic fields produced by alignment coils. It is necessary for the alignment of the electron beam with the focus field to be optimized to minimize lateral energy components in the beam.
The conventional technique of camera tube alignment consists of causing the electron beam to rotate with respect to the center of the target. Misalignment is manifest by the electron beam missing the center of the target, as indicated by the camera tube image. Alignment corrections are effected so as to reduce the error. This technique is generally inconvenient and inefficient.
In a camera tube an electron beam is scanned across the camera tube target so as to discharge the target and thereby provide picture element signals. The camera tube target is charged by radiation focused by a camera lens onto the camera tube target. In order to insure that the camera tube target is always fully discharged it is necessary to insure that the beam current is sufficiently high to discharge the target under operational peak light levels. This cannot be achieved in practice by simply increasing the incident light to effect a setting due to problems with color temperature scaling.
It is known to set the beam current by subjecting the camera tube to the operational peak light level and by reducing the scan speed so that the target charges to a higher level than that which would result under normal operational conditions. The beam current is adjusted so as to fully discharge the target under the adjustment conditions whereby full discharge of the target during normal operational conditions is insured.
The provision of circuitry for varying the scanning speed and the general implementation of this known method is complex and inconvenient.